RF power amplifiers are used in a variety of applications such as base stations for wireless communication systems, etc. The signals amplified by the RF power amplifiers often include signals that have a high frequency modulated carrier having frequencies in the 400 megahertz (MHz) to 60 gigahertz (GHz) range. The baseband signal that modulates the carrier is typically at a relatively lower frequency and, depending on the application, can be up to 300 MHz or higher. Many RF power amplifier designs utilize a semiconductor switching device as the amplification device. Examples of these switching devices include power transistor devices, such as a MOSFET (metal-oxide semiconductor field-effect transistor), a DMOS (double-diffused metal-oxide semiconductor) transistor, a GaN HEMT (gallium nitride high electron mobility transistor), a GaN MESFET (gallium nitride metal-semiconductor field-effect transistor), an LDMOS transistor, etc.
Class F amplifier configurations are gaining increased favor in modern RF applications due to their highly efficient operation. In class F operation, the input of the switching device (e.g., the gate) is modulated while a reference terminal of the switching device (e.g., the source) is maintained at a fixed potential. During the ON state of the switching device, voltage is nominally at zero across the output of the switching device while a sinusoidal switching current is present across the output terminal of the switching device. Conversely, During the OFF state of the switching device, current is nominally at zero across the output of the switching device while a half square wave voltage appears at the output terminal of the switching device. Theoretically, no power is dissipated because both states are characterized by zero IV. In practice, power dissipation occurs at the transition between ON and OFF states when there is an overlap between the current sine wave and the voltage square wave and hence current and voltage simultaneously appear at the output terminal. Highly efficient class F operation is obtained by minimizing this overlap.
One technique for minimizing the current-voltage overlap in class F amplifiers involves harmonic filtering. By mitigating harmonic oscillation at the output of the device, the shape the voltage and current waveforms is beneficially improved for minimal overlap. Nominally, the output of the amplifier should present a short circuit path to the even ordered harmonics (e.g., 2F0, 4F0, 6F0, etc) of the fundamental frequency F0, i.e., the frequency of the RF signal being amplified. In addition, the output of the amplifier should nominally present an open circuit to the odd ordered harmonics (e.g., 3F0, 5F0, 7F0, etc.) of the fundamental RF frequency F0.
Known techniques for harmonic tuning of Class F amplifiers include incorporating filters into the impedance matching networks that are connected to the input and output terminals of the amplifier device. These impedance matching networks can be provided on a printed circuit board (PCB) that accommodates the packaged amplifier device. Alternatively or in addition, filters can be incorporated into the packaged amplifier device itself by placing discrete capacitors and inductors between the integrated circuit and the package leads. In either case, the impedance matching networks can include LC filters that are tuned to the harmonics of the fundamental frequency F0 so as to provide an electrical short or open circuit, as the case may be. Instead of LC components, microstripline geometries, such as quarter wavelength transmission lines, open stubs, radial stubs, etc. may be used in the PCB to provide a desired frequency response.
One drawback of conventional harmonic tuning designs is that higher order harmonics become increasingly difficult to filter with increasing separation from the current source. For example, in the above described configurations, parasitic reactance of the package level and board level conductors substantially influences the propagation of higher frequency signals. As a result, the ability to tune out a third order harmonic, which may be in the range of 6 GHz in modern RF applications, is very limited at the package level or board level.